The manufacture of a number of products including paper, paper board, fibre board and non-woven fabrics involves a step in which the particles used to form the end product are suspended in a liquid transporting medium. It is often desirable to determine various physical parameters of the particles while they are suspended in the transporting liquid. For this purpose it is necessary to obtain a representative sample of the particles. Normally, only a sample of the particles and not the liquid is required. A particular example of the need to obtain a representative sample for analysis is to be found in the paper making industry. In this regard Australian patent application no. 48793/85 discloses a method and apparatus for making paper in which a sample of fibrous stock taken from a liquid suspension is analysed to provide information for adjustment of refining means used to process the stock.
An inherent difficulty confronts any attempt to obtain a representative sample of fibrous particles from a suspension thereof, due to the tendency of such particles to agglomerate by entanglement so as to block any orifice through which a sample of the suspension may be drawn. That difficulty may to a certain extent be overcome by agitating the bulk suspension by mechanical means to effect a mixing and disentanglement of the particles. However, even within an agitated vessel there may be eddies or currents which act to separate particles having different physical properties. A human operator can be used to offset this problem by taking a sample from an agitated part of the vessle, however, this introduces further problems of accessibility, safety and consistency of sampling technique. Moreover, this technique does not lend itself to automation so that continuous sampling can be effected.
Very often the liquid and particle mixtures of interest are pumped through pipelines during the various stages of production. At the center of such a pipeline the motion of the particles is relatively uniform, free of eddies and settling does not occur. Accordingly, a sample drawn from the central region of the pipeline will be relatively typical of the particles travelling in the stream of material. As the pressure within a pipeline conveying fluid is normally higher than external pressure, a small sampling aperture provided, for example, by means of a tube extending to the center of the pipeline should result in a sample being forced through the tube to the exterior of the pipeline. In practice however, unless the sampling aperture is very large compared to the dimension to the particles suspended in the liquid, bridging or blocking of the sampling aperture can occur. Any such bridging or blocking will result in the sampling becoming selective and not representative of the actual particle distribution in the liquid, or may interrupt sampling flow entirely.